Loud speaker



-Oat. 17, 1933.

. 1-1. L. KIRKE El Al. ,930,757

LOUD SPEAKER Filed Nov. 9, 1952 UNITED STATES PATENT. OFFICE LOUD SPEAKER Harold Lister Kirke, Clapham Park, London, Alfred Bernard Howe, Hatch End, and Arthur Edwin Barrett, England Clapham Common, London,

Application November 9, 1932, SerialNo. 641,908, f

and in Great Britain November 27, 1931 8 Claims.

Thepresent invention relates to loudspeakers.

In. loudspeakers comprising a vibratory sound radiating diaphragm it is known that, in order to obtain adequate response at thelower acoustic frequencies, it is necessary to make the length of the path by which pressure waves from the back of the diaphragm can reach the front of the diaphragm at least of the same order of magnitude as one'quarter of the wavelength of the' lowest frequency to be radiated. For this purpose diaphragms have been surrounded'by flat rigid baffies. It will be evident that flat baflles, to be efficient, must be of considerable size and therefore cumbersome.

It has also been proposed, therefore, to mount a loudspeaker diaphragm in an aperture in the front wall of a. box and insome cases the back of the box has been left open. The inside of such boxes has also been lined with sound damping material such as felt. i

Loudspeakers of this kind. suffer from so-called box resonance effects due to reflections from the walls of the boxand also to resonance of the air column enclosed within the box. The thickness of the lining :of sound damping material hitherto used has been far too small to prevent resonance due to wall reflections and inany case there has been air column resonance'due to reflections from the plane of the back of the box where there is a sudden transition from an enclosed air column to the free air. This'air column resonance is selective, that is to say it occurs at one or more definite frequencies and the response at these frequencies is thereby unduly increased (or in the case of an anti-resonance decreased) at the expense of neighbouring frequencies.

It has also been proposed to fill the space behind the diaphragm With damping material, but this leads to lack of sensitivity at low frequencies due to excessive damping of the movement of the diaphragm at such frequencies.

It is an object of the present invention to provide improved means whereby the length of the path between the front and the back of the diaphragm may be made relatively largewithout the use of a bafile'of large frontal area and without introducing appreciable selective box resonance effects.

According to the present invention, there is provided upon one side of a diaphragm, such as the sound radiating diaphragm of a loudspeaker, an air chamber wholly or partially bounded by sound damping material of such nature and thickness that a large proportion of the sound energy of lower frequencies falling upon the material is 'phragm,

bodiment in Fig. 1,

Figs. 3 and 4 are views, corresponding to those of Figs. 1 and 2, of a further embodiment of the invention.

' Referring to Figs. 1 or 2, the diaphragm 1 of a loudspeakeriis mounted in knownmanner in an aperture in the front wall 2 of a box, the back of the box being left open. The inside of the box is lined with a thick layer of damping material 3, forexample the material known as slag wool or rock wool. This material is of a fibrous nature and is kept in place by means of a former 4 constructed of wire mesh covered with thick muslin to retain' the fine particles of the wool and yet to offer little obstruction to the passage of sound Waves. The cross-sectional area of the air chamber increases approximately logarithmically from thediaphragm outwards. Thus, the thickness of the damping material at the smaller end of the air chamber is equal to the distance of the edge of the diaphragm from the inside of the box, and may be comparable with the diaphragm diameter for example, and the thickness flaresjdown to nothing at the end remote from the diaphragm. The thickness is therefore greatest where the sound pressures are highest and resonance due to reflection from the side walls of the chamber is substantially prevented. Further, owing to the flared shape of the air column behind" the dia-, phragm, the air column resonance is made much less selective of particular frequencies than if it were'not flared and the flaring also reduces resonance due to reflections from the mouth of the air chamber.

A further advantage of the association of a" flared air column of the kind described with a diaparticularly when the diaphragm mounting is such that the resonant frequency of the mass of the diaphragm and the stiffness of its mounting is low, is that the air column pro vides a further range of stiffnesses which, associated with the diaphragm mass, provide resonant frequencies is usually between the fundamental natural frequency of vibration of the diaphragm on its mounting and the middle frequencies. Since this resonance is provided over a range of frequencies, it is not objectionable as in the case of selective resonance, that is to say relatively sharp resonance at one or more distinct frequencies.

If desired an air column of the kind described may be provided upon both sides of the diaphragm. The rates of flare and the inlet areas of the two chambers may be the same or they may be made different.

The back of the air chamber may be covered either with a thick layer of damping material or with material which does not appreciably affect the passage of sound waves, such as thin fabric.

In another arrangement shown in Figs. 3 and 4, the top and bottom of the box are omitted so that there remain the front and the two sides. These sides are lined with damping material to form a trough shaped air chamber increasing in width from the front of the box backward. The thickness of the damping material at the front wall 2 may be made greatest in the neighbourhood of the diaphragm as indicated by the dotted lines in Fig. 3.

With this arrangement, the height of the structure is preferably made considerably greater than the breadth in view of the absence of the top and bottom and, if desired, the diaphragm may be arranged nearer the bottom than the top so that the advantage is taken of the floor acting as the bottom of the box. Further, the sides of the structure may be hinged to the front, somewhat as in a fire screen, or they may be fixed at an obtuse angle to the front.

We claim:

1. A device for association with a sound radiating diaphragm comprising a body of sound damping material having a flaring passage therethrough and a lining for said passage, said lining offering little obstruction to the passage of sound waves, and being adapted to retain said material in position, the thickness of said material at the smaller end of said passage being not less than one-half the diameter of the smaller end of said passage and decreasing substantially to nothing at the larger end of said passage.

2. A device for association with a sound radiating diaphragm comprising a body of sound damping material having a substantially logarithmically flaring passage therethrough and a lining for said passage, said lining offering little obstruction to the passage of sound waves, and being adapted to retain said material in position, the thickness of said material at the smaller end of said passage being not less than one-half of the diameter of the smaller end of said passage.

3. A loud speaker cabinet comprising a substantially rectangular box having an apertured front wall, and a body of sound damping material in said box, said body having its inner surface shaped to form an air chamber increasing progressively in diameter in a rearward direction from the aperture in said front Wall, said body having its outer surface adjacent the interior surface of the walls of said box.

4. An acoustic instrument comprising a-box having an apertured front wall and four side walls, a diaphragm vibratably mounted in the aperture in said front wall and, upon the inside of said side walls, sound damping material the thickness of which decreases from the neighbourhood of the diaphragm backwards.

5. An acoustic instrument comprising a box having an apertured front wall and four side Walls, a frusto-conical diaphragm vibratably mounted in the aperture in said front wall, actuating means for said diaphragm located within said box and upon the inside of said side walls, sound damping material the thickness of which decreases from the neighbourhood of the diaphragm backwards.

6. An acoustic instrument comprising a structure having an apertured front wall and two side walls, a diaphragm vibratably mounted in the aperture in said front wall and, upon the inside of said side walls, sound damping material the thickness of which decreases from the neighbourhood of the diaphragm backwards.

7. An acoustic instrument comprising a box having an apertured front wall and four side walls, a diaphragm vibratably mounted in the aperture in said front wall and, upon the inside of said side walls, fibrous sound damping material the thickness of which decreases from the neighbourhood of the diaphragm backwards, said damping material being covered with a material adapted to retain said damping material and to offer little obstruction to the passage of sound waves.

8. An acoustic instrument comprising a structure having an rectangular apertured front wall of greater height than breadth and two side walls one associated with each of the longer sides'of said front wall, a diaphragm vibratably mounted in the aperture in said front wall and, upon the inside of said side Walls, sound damping material the thickness of which decreases from the neighbourhood of the diaphragm backwards.

HAROLD LISTER KIRKE. ALFRED BERNARD HOWE. ARTHUR EDWIN BARRETT. 

